1. Field of the Invention
The present invention relates to an image formation apparatus which forms images by transferring a toner image formed on an image carrying member onto a transfer medium.
2. Description of the Related Art
Hitherto, there have been proposed image formation apparatuses such as electrostatic photocopiers and printers and the like which form images based on image information converted into digital signals, and some arrangements of such image formation apparatuses comprise an image carrying member upon which an electrostatic latent image is formed, a developing device for developing the electrostatic image formed on the image carrying member using a developing agent so as to form a developing agent image, and a transfer device for transferring the developing agent image formed on the image carrying member onto a transfer medium.
FIG. 7 is a schematic diagram illustrating the interior configuration of such a conventional image formation apparatus, wherein reference numeral 101 denotes a photosensitive drum, which comprises a photosensitive member formed of amorphous silicon or the like on the surface thereof, and which serves as an image-carrying member for forming an electrostatic latent image, 102 denotes a primary charger for charging the surface of the photosensitive drum 101, 104 denotes a developing device which applies voltage between a developing sleeve 104a and the photosensitive drum 101 to develop the electrostatic latent image with toner serving as a developing agent, thereby forming a toner image which is the developing agent image formed on the photosensitive drum 101, and 110 denotes a pre-transfer charger for making uniform a charge of the toner image formed on the photosensitive drum 101, 108 denotes a transfer belt for holding and transporting a transfer medium P in contact with the photosensitive drum 101, 113 denotes a transfer blade which applies a current from the back side of the transfer belt 108 so as to transfer the toner image formed on the photosensitive drum 101 onto the transfer medium P, 105 denotes a separating charge for separating the transfer medium P from the transfer belt 108, 111 denotes a cleaning device which removes toner residually adhering to the photosensitive drum 101 instead of being transferred onto the transfer medium P (untransferred residual toner), using a cleaning blade 111a, 106 denotes a fixing device for fixing the image formed on the transfer medium P thereupon, and 112 denotes a semiconductor layer for casting a laser beam 123 modulated by image signals, wherein the laser beam 123 cast from the semiconductor laser 112 is reflected by a rotating polygonal mirror 114 and then passes through an imaging lens 116 and reflected by a reflecting mirror 115, so as to perform raster scanning on the surface of the photosensitive drum 101.
With such an image formation apparatus configuration, a toner image is first formed applying the image-creating processes of charging, exposing, and developing on the photosensitive drum 101, followed by transferring of the toner image onto the transfer medium P, and subsequently fixing the toner image so as to obtain an article with an image formed thereupon.
The cleaning blade 111a cleans the surface of the photosensitive member by rubbing the surface thereof, and the residual toner and any external additives on the surface of the photosensitive drum 101 following the transfer processes play an important role in increasing the lubricity between the cleaning blade 111a and the photosensitive member, polishing the photosensitive member, and suitably grinding the photosensitive member.
The effects thereof are great in the event of using an external additive effective in increasing lubricity such as titanium oxide or alumina fine powder, or using an external additive effective in increasing polishability such as strontium titanate powder, ceric oxide powder, calcium titanate powder, and so forth, for example.
That is to say, these external additives are effective in increasing the lubricity on the surface of the photosensitive member, polishing the photosensitive member, and suitably grinding the photosensitive member. Also, these external additives serve to prevent deterioration of image quality due to substances generated by discharge of the chargers 102 and 110, or components in the toner, adhering to the surface of the photosensitive member, and to prevent deterioration of the slidability of the cleaning blade due to an increase in the friction coefficient on the surface of the photosensitive member.
However, some of these external additives have weak adhesion as to the toner, and some have a polarity which is inverse to a friction charging polarity of the toner, so in the event that the developing device 104 is operated at any time other than when forming an image, this results in the external additives scattering off of the toner on the developing agent holding member (developing sleeve), and even in the event that an electrical field is formed between the developing agent holding member and the photosensitive member so that the toner does not adhere to the photosensitive member, the external additives charged with the inverse polarity may depart from the toner and adhere to the photosensitive member. Accordingly, operating the developing device at times other than when forming images causes the amount of external additives within the developing agent to decrease.
In the event that the amount of external additives within the developing agent in the developing device decreases, the amount of external additives adhering to the photosensitive member along with the toner also decreases, so the amount of external additives sent to the cleaning device 111 as untransferred residue decreases.
In the event that the amount of external additives sent to the cleaning device 111 decreases thus, the lubricity with the surface of the photosensitive member decreases, and the effects of polishing and suitably grinding the photosensitive member decrease, leading to deterioration of image quality due to substances generated by a discharge from the chargers, or components in the toner, adhering to the surface of the photosensitive member.
Also, the friction coefficient on the surface of the photosensitive member rises due to the adhering substances, leading to poor slidability of the cleaning blade 111a, which further leads to chattering of the cleaning blade 111a or eversion thereof. Such problems occur particularly readily with photosensitive drums which are not readily ground, such as a-Si drums, since the substances adhering to the drums are not readily removed.
Such problems also occur in the event that the time in which the developing device operates during non-image-formation periods is long, for example, and the time of the developing device operating during non-image-formation periods becomes long in cases such as forming continuous images with a transfer medium having a great basis weight, such as heavy paper or the like.
The reasons for these problems are as follows: in the event of carrying out the fixing process consecutively on a transfer medium having a great basis weight, the fixing speed generally must be slowed since defective fixing readily occurs due to the temperature of the fixing device dropping, which in turn leads to more spacing between the transfer mediums (hereafter also referred to as xe2x80x9csheet spacingxe2x80x9d). This slowdown is to prevent a subsequent transfer medium from colliding with a transfer medium undergoing fixing in the event that the fixing speed is slower but the image formation speed is not changed, which could lead to faulty transportation and improper images. Or, in the event that both the image formation speed and the fixing speed are made to be slower, the amount of time elapsed is longer even in the event that the sheet spacing is the same as before the change in speed. Due to such reasons, the sheet spacing time becomes greater, and consequently, the amount of time wherein the developing device is being operated in the non-image-formation period becomes longer. As a result, the cleaning capabilities decrease, which has been a problem.
Japanese Patent Laid-Open No. 9-152793 discloses an arrangement wherein transfer bias is applied to the margin of the leading side of the sheet following the leading edge thereof reaching the transfer position, and wherein the transfer bias is turned off at the margin on the trailing side before the trailing edge of the sheet reaches the transfer position, in order to prevent charging of the photosensitive drum due to transferring while preventing faulty transfer at the leading and trailing edge portions of the toner image.
Also, Japanese Patent Laid-Open No. 2000-72705 discloses an arrangement wherein bias with the same polarity as transferring is applied in the event that the transfer medium is not present in order to prevent the back side of the transfer medium from becoming soiled due to toner charged with an inverse polarity, thereby preventing inverse-polarity toner from migrating from the photosensitive drum side to the transfer member.
However, these documents neither disclose nor make any mention of the problem of unstable cleaning conditions due to a decrease in the external additives within the toner, much less either disclose or mention a method to deal with the problem.
The present invention has been made in light of the above-noted problems, and accordingly it is an object thereof to provide an image formation apparatus capable of effectively removing foreign matter on the image holding member without deterioration of image quality.
To this end, according to a first aspect of the present invention, an image formation apparatus comprises: image formation means for forming a toner image on an image carrying member; transporting means for transporting a transfer medium; transfer means for electrostatically transferring the toner image onto a transfer medium transported by the transporting means to a transfer unit; cleaning means for cleaning residual matter including toner from the surface of the image carrying member; bias applying means for applying a bias of a polarity which is inverse to a normal polarity of the toner, to the transfer means; and control means for controlling operations of the bias applying means; wherein, in the event of forming images on multiple transfer mediums in a continuous manner, the control means places the bias applying means in an inoperative state at a first timing before a trailing edge of a preceding transfer medium reaches the transfer unit, places the bias applying means in an operating state at a second timing following the trailing edge of the preceding transfer medium passing through the transfer unit, places the bias applying means in an inoperative state at a third timing before a leading edge of a subsequent transfer medium reaches the transfer unit, and places the bias applying means in an operating state at a fourth timing following the leading edge of the subsequent transfer medium passing through the transfer unit.
According to a second aspect of the present invention, an image formation apparatus comprises: image formation means for forming a toner image on an image carrying member; transporting means for transporting a transfer medium; transfer means for electrostatically transferring the toner image onto a transfer medium transported by the transporting means to a transfer unit; cleaning means for cleaning residual matter including toner from the surface of the image carrying member; bias applying means for applying a bias of a polarity which is inverse to a normal polarity of the toner, to the transfer means; bias application operations control means for controlling operations of the bias applying means; fixing means for fixing the toner image transferred to the transfer medium; and transfer medium spacing control means capable of changing the transfer medium spacing time from a trailing edge of a preceding transfer medium passing through the transfer unit until a leading edge of the subsequent transfer medium is transported to the transfer unit, in the event of forming images on multiple transfer mediums in a continuous manner, wherein the bias application operations control means enables operation of the bias applying means in the transfer medium spacing time according to the transfer medium spacing time.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).
FIG. 1 is a schematic diagram of the internal configuration of an image formation apparatus according to an embodiment of the present invention.
FIG. 2 is an enlarged diagram of the transfer unit of the image formation apparatus.
FIG. 3 is a diagram illustrating the position and relation between the transfer bias in the transfer unit and the transfer medium.
FIG. 4 is a Table showing the results of image formation, and so forth, with the image formation apparatus.
FIG. 5 is a flowchart illustrating a second embodiment of the present invention.
FIG. 6 is a diagram of another example of the image formation apparatus.
FIG. 7 is a schematic diagram illustrating the internal configuration of a conventional image formation apparatus.